One of the major advances in communications in recent years has been the increased use of optical fiber systems for carrying large quantities of information with low distortion and low cost over great distances.
Optical delay lines are important components in optical fiber communication systems. They are critical components in switching streams of optical signals from one path to another. They are used in packet switching for Internet applications, and they are building blocks in the correlation filters used to recognize packet addresses. Delay lines are also used in the buffers that control the timing of signal routing. And they are used for synchronizing data bits, for compensating dispersion between different optical paths, and for compensating differential delay between paths.
Variable delay lines are needed because different delays may be required at different times. In addition to variation required by the dynamics of optical network components, variation of delays in the optical network may be required to compensate path length differences in RF links in the network such as differences due to changes in a satellite orbit, due to the use of multiple ground antennas, or due to atmospheric scintillation or beam squinting.
A variable optical delay line typically comprises a plurality of optical fibers, each cut to a slightly different length, and a fiber switch to select the fiber path, thereby selecting the delay time. In principle this should work, but in practice it has serious shortcomings in high resolution systems. Cutting fibers to a precision of millimeters is difficult, and this difficulty limits the achievable accuracy of delay time. Moreover, a variable delay line having a large dynamic range will require many precisely cut fibers. It is not practical to provide sufficient precisely cut fibers to provide fine delay increments over a large dynamic range. Ideally the variable delay line is also compact. It is difficult to achieve compact structures of many fibers of different lengths.
In addition is the problem of switching an input signal from one fiber to another. Ideally switching is “hitless” in that no transmitted data is lost. This may be difficult to achieve in switching a beam from one fiber to a nonadjacent other fiber. Accordingly there is a need for an improved variable optical delay line that is compact, provides high resolution over a large dynamic range and is preferably hitless.